What you needChunky Foam
Marble Spray Paint
This project uses marble paint that comes in a spray can and is available from your hardware store. If you can not find the marble spray paint then use a grey stone paint and sponge the other textures onto the stone using black and white until it looks like a rock.
Take your piece of foam and using some scissors cut it into a rock like chunk. Spray the foam using the spray paint. Allow it to dry and give it a second or third coat if required.
Using a skewer push a hole into the rock to form the hole for the keychain. Thread the chain through the hole and clip it back onto the chain to form a loop/knot as shown in the picture.
You can recycle old foam from mattresses or chairs to reduce the cost of this project.